The present invention relates to a liquid crystal display device and, more particularly, to a technique which is applicable to a TFT (Thin Film Transistor) type of liquid crystal display device made of polysilicon transistors.
An active matrix type liquid crystal display device which has an active element for each pixel and causes the active element to perform a switching operation is one known type of liquid crystal display device. A TFT type of active matrix liquid crystal display module which uses as its active elements thin film transistors each made of an amorphous silicon MOS transistor or a polysilicon MOS transistor is a known type of this active matrix type liquid crystal display device. In the following description, a amorphous silicon MOS transistor will be referred to as an xe2x80x9camorphous-SiTrxe2x80x9d, a polysilicon MOS transistor will be referred to as a xe2x80x9cPoly-SiTrxe2x80x9d, a TFT type liquid crystal display module using amorphous silicon MOS transistors will be referred to as an xe2x80x9camorphous-SiTr-TFT liquid crystal display modulexe2x80x9d, and a TFT type of liquid crystal display module using polysilicon MOS transistors will be referred to as a xe2x80x9cPoly-SiTr-TFT liquid crystal display modulexe2x80x9d.
The amorphous-SiTr-TFT liquid crystal display module is widely used as a display device for personal computers or television sets. However, in an amorphous-SiTr-TFT liquid crystal display module, a driver circuit for driving the liquid crystal needs to be provided at the periphery of the liquid crystal display panel.
In recent years, a TFT type of liquid crystal display module using Poly-SiTr elements has been developed and is now used in liquid crystal projectors, head-mounted (glasses-type) displays and the like. In the liquid crystal display panel of a Poly-SiTr-TFT liquid crystal display module, Poly-SiTr elements are formed so as to be disposed in matrix form on a quartz or glass substrate, as in the case of the liquid crystal display panel of an amorphous-SiTr-TFT liquid crystal display module.
Moreover, since the operating speed of the Poly-SiTr element is faster than that of the amorphous-SiTr element, the liquid crystal panel of the Poly-SiTr-TFT liquid crystal display module can be formed on one substrate together with its peripheral circuits. This is described in, for example, NIKKEI ELECTRONICS, Nikkei-McGraw-Hill, Feb. 28, 1994, pp. 103-109.
In the existing type of single-crystal Si semiconductor MOS transistor, even with a comparatively simple circuit construction, such as that shown in FIG. 14, it is possible to avoid, at a practical level, the shift of the voltage level of a threshold voltage Vth of each MOS transistor TR1 to TR3. However, under the present circumstances, in a Poly-SiTr element whose channel formation region is made of polycrystalline silicon, multiple grain boundaries arc generally present below its gate. Accordingly, even if transistors each having the same dimensions arc disposed in the vicinity of one substrate, their threshold voltages Vth generally do not coincide to an extent that can be practically approximated. Therefore, in general, in the case of a circuit construction such as that shown in FIG. 14, which uses Poly-SiTr elements, output voltages VOUT1 to VOUT3 of the respective MOS transistors TR1 to TR3 shift to an unallowable extent in practical terms.
If, for example, Poly-SiTr elements are used and a circuit construction such as that shown in FIG. 14 is adopted for the purpose of supplying pixel drive voltages (or grayscale voltages) to the respective pixels of a liquid crystal panel of a Poly-SiTr-TFT liquid crystal display module, there is the problem that linear patterns occur on the display screen of a liquid crystal display panel owing to the shifts of the output voltages VOUT1 to VOUT3 due to the shifts of the threshold voltage (Vth) of the respective Poly-SiTr elements, so that the display quality of the display screen of the liquid crystal display panel is impaired to a remarkable extent.
The present invention has been made to solve the problem of the above-described related art, and an object of the present invention is to provide a technique which is capable of improving the display quality of the display screen of a liquid crystal display element in a liquid crystal display device.
The above and other objects and novel features of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings.
Representative aspects of the invention disclosed in the present application are as described below in brief.
The present invention provides a liquid crystal display device which comprises plural pixels provided in matrix form, plural video signal lines which apply pixel drive voltages to pixels arrayed along columns or rows of the matrix of the plural pixels, and a drive part which supplies the pixel drive voltages to the plural video signal lines. The drive part includes plural video signal input parts which supply the pixel drive voltages to the respective video signal lines. Each of the video signal input parts includes a first field-effect transistor, a first part which sets a voltage value of a control electrode of the first field-effect transistor to a voltage value obtained by correcting a common pixel drive voltage by a threshold voltage of the first field-effect transistor, a second part which sets the voltage value of the control electrode of the first field-effect transistor to a voltage obtained by adding a video signal voltage to the voltage value corrected by the first part, and a third part which supplies a voltage obtained by adding the video signal voltage to the common pixel drive voltage, to the video signal line as a pixel drive voltage, as well as to the first field-effect transistor, the voltage value of whose control electrode is set by the second part to the voltage obtained by adding the video signal voltage to the voltage value corrected by the first part.
According to the present invention, the drive part has a control part which controls each of the video signal input parts, and the control part transmits a first-mode control signal to each of the video signal input parts and causes each of the video signal input parts to supply a voltage, obtained by adding the video signal voltage to the common pixel drive voltage, to the corresponding one of the video signal lines as the pixel drive voltage, and also transmits a second-mode control signal to each of the video signal input parts and causes each of the video signal input parts to supply a voltage, obtained by subtracting the video signal voltage from the common pixel drive voltage, to the corresponding one of the video signal lines as the pixel drive voltage.
According to the present invention, the first-mode control signal transmitted from the control part has first to fifth control signals, and the first to fifth control signals are transmitted to each of the video signal input parts in the order of from the fifth control signal to the fourth control signal to the third control signal, and in the order of from the first control signal to the second control signal, while the fifth control signal is being transmitted.
According to the present invention, the second-mode control signal transmitted from the control part has first to fifth control signals, and the first to fifth control signals are transmitted to each of the video signal input parts in the order of from the fourth control signal to the first control signal to the second control signal to the fifth control signal to the third control signal.
According to the present invention, the first part includes a second field-effect transistor having a second electrode to which a first reference voltage is to be applied, and a first electrode connected to the control electrode of the first field-effect transistor, a third field-effect transistor having a second electrode connected to the first electrode of the second field-effect transistor and a first electrode connected to the second electrode of the first field-effect transistor, and a fourth field-effect transistor having a second electrode connected to the first electrode of the first field-effect transistor and a first electrode to which the common pixel drive voltage is to be applied. The third part includes a fifth field-effect transistor having a second electrode connected to a second reference voltage and a first electrode connected to the second electrode of the first field-effect transistor, and a sixth-field-effect transistor having a second electrode connected to the first electrode of the first field-effect transistor and a first electrode connected to the corresponding one of the video signal lines. The second field-effect transistor is turned on when the first control signal outputted from the control part is applied to a control electrode of the second field-effect transistor. The third and fourth field-effect transistors are turned on when the second control signal outputted from the control part is applied to control electrodes of the respective third and fourth field-effect transistors. The fifth and sixth field-effect transistors are turned on when the third control signal outputted from the control part is applied to control electrodes of the respective fifth and sixth field-effect transistors.
According to the present invention, the second part includes a seventh field-effect transistor having a second electrode to which a video signal voltage is to be applied, an eighth field-effect transistor having a first electrode to which a third reference voltage is to be applied, and a second electrode connected to a first electrode of the seventh field-effect transistor, and a coupling capacitor connected between the first electrode of the seventh field-effect transistor and the first electrode of the second field-effect transistor. The seventh field-effect transistor is turned on when the fourth control signal outputted from the control part is applied to a control electrode of the seventh field-effect transistor, and the eighth field-effect transistor is turned on when the fifth control signal outputted from the control part-is applied to a control electrode of the eighth field-effect transistor.
According to the present invention, the second part includes plural data input parts provided by the number of bits of display data, and each of the data input parts includes a latch part which stores each bit value of display data, a seventh field-effect transistor having a second electrode connected to the latch part, an eighth field-effect transistor having a first electrode to which a third reference voltage is to be applied, and a second electrode connected to a first electrode of the seventh field-effect transistor, and a coupling capacitor connected between the first electrode of the seventh field-effect transistor and the first electrode of the second field-effect transistor. The seventh field-effect transistor of each of the data input parts is turned on when the fourth control signal outputted from the control part is applied to a control electrode of the seventh field-effect transistor. The eighth field-effect transistor of each of the data input parts is turned on when the fifth control signal outputted from the control part is applied to a control electrode of the eighth field-effect transistor.
According to the present invention, the drive part includes two lines of video signal input parts, and further includes plural selecting parts which alternately supply pixel drive voltages from the two lines of video signal input parts to the corresponding one of the video signal lines.
According to the present invention, in each of the field-effect transistors, a channel formation region below the control electrode is made of polycrystalline silicon. According to the present invention, the plural pixels provided in matrix form, the plural video signal lines and the drive parts are incorporated in a liquid crystal display element.